The present invention generally relates to systems and methods relating to the torrefaction of lignocellulosic material.
Torrefaction refers to the thermal treatment of wood, usually in an inert atmosphere, at relatively low temperatures of 225 to 300° C. Torrefaction generally results in a fuel with increased energy density relative to the mass, by the decomposition of reactive hemicellulose content of the wood.
Wood generally contains hemicellulose, cellulose, and lignin. In an aspect, the goal of torrefaction is to remove moisture and low weight organic volatile components from the wood. Torrefaction may also depolymerize the long polysaccharide chains of the hemicellulose portion of the wood and produce a hydrophobic solid product with an increased energy density (on a mass basis) and improved grindability. Because of the change in the chemical structure of the wood after torrefaction, it can be suitable for use in coal fired facilities (torrefied wood or biomass has the characteristics that resemble those of low rank coals) or can be compacted into high grade pellets replacing standard wood pellets.
Torrefaction has developed over the last few decades as a possible method to turn wood based biomass into a viable addition to the spectrum of energy products. Although there has been much research into the compositional changes that occur in the biomass (wood) while undergoing torrefaction, commercial processes are not well developed. The torrefaction method and system put forth here has been developed to meet the commercial need for a viable torrefaction process. Other torrefaction processes are described in: U.S. Patent Pub. No. 2008/0223269, in which conduction heat is used to achieve torrefaction; U.S. Pat. No. 4,787,917, in which torrefied wood is formed into sticks of unbarked wood; and PCT Pub. No. WO 2005/056723, in which a continuous method and system produces torrefied biomass from raw material (organic material and originate from forestry or other agriculture and material of fossil nature or mixture—lignocellulose).